Apparatus for controlling coating thickness



Oct. 22, 1968 R. w. PATTERSON 3,406,656

APPARATUS FOR CONTROLLING COATING THICKNESS a fi I HVVE/V TOR.

ROBERT W. PATTERSON M M Attorney Oct. 22, 1968 R. w. PATTERSON APPARATUS FOR CONTROLLING COATING THICKNESS 4 Sheets-Sheet 2 Filed Feb. 2, 1967 INVEN TOR. ROBERT W. PATTERSON 5y W Attorney R. w. PATTERSON 3,406,656

APPARATUS FOR CONTROLLING COATING THICKNESS Oct. 22, 1968 Filed Feb. 2, 1967 4 Sheets-Sheet 5 INVENTOR. ROBERT W. PATTERSON Attorney Oct. 22, 1968 R. w. PATTERSON 3,406,656

APPARATUS FOR CONTROLLING COATING THICKNESS Filed Feb. 2, 1967 4 Sheets-Sheet 4.

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Pass Line lNVE/V TOR. ROBERT W. PATTERSON A Horney United States Patent 3,406,656 APPARATUS FOR CONTROLLING COATING THICKNESS Robert W. Patterson, Birmingham, Ala., assignor to United States Steel Corporation, a corporation of Delaware Filed Feb. 2, 1967, Ser. No. 613,476 6 Claims. (Cl. 118-63) ABSTRACT OF THE DISCLOSURE An air knife with a curved orifice slot useful in systems for controlling the weight and distribution of a coating applied to a moving or continuous substrate. The curved air knife enables formation of a coating on a substrate having a convex profile which facilitates coiling.

This invention relates to an apparatus for use in controlling the weight and distribution of a coating applied to a moving substrate. More particularly, the invention relates to an apparatus for use in a process wherein a substrate to be coated is immersed into and withdrawn from a bath of coating material. The apparatus is particularly suited for use in hot-dip coating operations wherein a substrate such as a steel strip is coated with a metal such as zinc, aluminum, tin or lead, and alloys thereof.

The present invention provides an apparatus for accomplishing control of the weight and distribution of a coating applied on a moving substrate which permits attainment of a convex cross section of a coated substrate. This cross section has the advantage that it facilitates coiling of the coated strip by avoiding inequalities in the thickness of the coating across the strip which might render coiling difiicult. Moreover, a coating of a convex cross section, in addition to being ideally suitable for coiling, completely eliminates edge damage which may result from such metal at the edge due to stretching and deformation during coiling.

Practical systems useful commercially have recently been proposed for controlling the weight of a coating applied to a substrate by means of a blast, i.e, stream or jet, of a fluid such as air or other gas projected against a coating before it solidifies. Such systems employ fluid nozzles referred to perhaps improperly but nevertheless commonly as air-knives, although obviously capable of use with many fluids other than air.

Heretofore, air knives used for coating control in continuous coating operations have been made with longitudinally straight orifice lips defining a rectangular orifice opening. It is apparent that the distance of travel of the fiuid stream from a straightdipped knife to an article will be the same at all points across the width of the strip. It is also apparent that increasing the angle of inclination will increase the length of fluid travel. Such streams may produce substantially rectangular cross sections of a coated substrate, but more characteristically, may result in a slightly concave cross section. It is desirable, however, to achieve a slightly convex cross section and this is achieved by the use of a curved air knife in accordance with the invention.

The advantages of the invention described above and others will be made more apparent from the following description taken in conjunction with the drawings.

In the drawings:

FIGURE 1 is an elevation view showing an embodiment of the apparatus in accordance with the invention;

FIGURE 2 is a side elevation of the apparatus shown in FIGURE 1;

FIGURE 3 is a top plan view of one of the air knives shown in FIGURE 1;

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FIGURE 4 is a front view of the air knife shown in FIGURE 3;

FIGURE 5 is a sectional view of the air knife shown in FIGURES 3 and 4 taken along lines VV of FIG- URE 4;

FIGURE 6 is a sectional View of the air knife taken along line VI-VI of FIGURE 4;

FIGURE 7 is a sectional view of the air knife taken along the lines VII-VII of FIGURE 4;

FIGURE 8 is a top plan view, similar to FIGURE 3, of an alternative embodiment of the air knife;

FIGURE 9 is an enlarged view of the means to adjust the inclination and horizontal position of the air knife, taken along lines IXIX of FIGURE 2;

FIGURE 10 is a plan view of the adjusting means taken along lines XX of FIGURE 9; and

FIGURE 11 is an end view of the air knife showing the angle of inclination as it might appear in use.

The elevation view of FIGURE 1 shows an arrangement in accordance with a preferred embodiment of the invention utilizing the curved air knife assembly in a conventional steel strip galvanizing line. In this illustration, a steel strip passes from an annealing furnace 2 through a snout 4 of controlled atmosphere into a receptacle 6 of coating material 8 which in the galvanizing operation is, of course, molten zinc. The strip leaves the bath in a vertical pass line defined by sink roll 10 and a deflector roll, not shown. As the strip moves upward from the coating bath, it passes gas nozzle assemblies 12 which effectively control the coating weight and distribution on the steel strip and a cooling box 14 which serves to bring the strip temperature below the melting point of the coating, i.e., zinc, to solidify the molten coating. A deflector roll, not shown, referred to above is located at the top of a cooling tower. The strip goes from the cooling tower to a conventional drive bridle, also not shown, which serves to pull the strip through the coating line.

The air knife assemblies 12 may be supported above the coating vessel 6 in any convenient manner; however, one suitable arrangement is shown in FIGURES l and 2. In this embodiment, a framework 22 of structural steel supports the assembly and other components. Each air knife assembly is provided with a structural support 24 and other supporting members, as shown. Each knife is also provided with individual means 26 for vertically adjusting its position with respect to the coating bath and with means 28 for horizontal adjustment with repsect to the surface of the strip, i.e., the pass line. The vertical positioning of the knives may be achieved manually by a hand crank and worm gear-ratchet combination as shown or automatically, i.e., electrically, pneumatically, etc. The means 28 for adjusting the horizontal position of the air knives depends from a beam 29 which in turn is suspended from the main support structure 24 by the vertical position adjusting means 26. Flexible connections, not shown, are used to supply fluid to the air knife assemblies 12.

The structure of the knife itself is best seen in FIG- URES 3 through 8. The air knife comprises a header section 30, a convergent throat or transition section 32 and a nozzle or orifice section 34. The orifice section includes upper and lower lips 36 and 38, respectively, defining a continuous slot-like orifice of height S. The lips extend longitudinally beyond the strip edges so as to provide an orifice wider than the widest strip to be processed in the apparatus. One of the lips, e.g., 36, may be adjustable to permit adjustment of orifice height S.

Disposed in the convergent throat section 32 and extending the full length of the orifice section 34 is a fine mesh wire or perforated plate or screen 40. Since it is necessary for satisfactory coating control that the air jet issuing from the air knife orifice be uniform throughout its length, a diffusing member such as a screen is used to affect a 90 change in the direction of air travel from the header and achieve a uniform air pressure across the convergent throat section 32. The screen accomplishes this function by providing a multiplicity of small openings having relatively high resistance to air flow from the header section 30. Uniformity of the flow profile of the fluid jet is also promoted by supplying fluid to the header section to both ends, i.e., doubleentry feed, via air inlets a and 30b. The inlets may conveniently be short pieces of pipe (30a and 30b) of suitable diameter which permit the knife to be rotatably mounted as will be hereinafter discussed.

The header section 30 is joined to the convergent throat section 32 by companion flanges 33 and 35. This arrangement permits the aforementioned airflow resistance screen 40 to be mounted in a suitable frame 40a and replaceably disposed across the opening into convergent throat section 32. The resistance screen may comprise aZOO-mesh screen woven from .0021 gauge stainless steel wire or a perforated plate of equivalent openings which provides about /3 open area and is satisfactory both as to strength and air flow resistance. Lips 36 and 38 are provided with connecting flanges 37 and 39 adapted to be bolted to flange 41 of the convergent throat section.

The curvature of the air knife according to One embodiment of the invention is shown by the displacement of the orifice in FIGURES 5, 6 and 7 in accordance with the position in FIGURE 4 from which the cross sections of FIGURES 5, -6 and 7 are taken. In the embodiment illustrated, the orifice is curved downwardly; thus, the cross section of the middle of the air knife of FIGURE 4 has the orifice positioned in its lowermost location and this is illustrated in FIGURE 5. The location of the orifice becomes higher in cross section toward the end of the knife and in FIGURE 6 the orifice is shown positioned about the middle while in FIGURE 7 the orifice is located in its uppermost position.

An alternative embodiment of the curved knife is shown in FIGURE 8. The curvature of the lips 34' may extend in a horizontal plane, i.e., away from the pass line and toward the header 30, and perform in a similar manner to the knife configuration depicted in FIGURES 3 through 7. Thus, the distance of the fluid travel in both embodiments is greater at the middle of the knife than at the ends and a convex profile or cross section may be developed on the coated substrate.

As mentioned above, for many applications the air knife is inclined from the horizontal toward the pass line at a positive or negative angle to a plane normal to the pass line. Adjustment of the inclination is achieved by rotation of the header and nozzle assembly in claimp 50 by loosening nut 43 and setscrew 51 as is seen in FIG- URES 9 and 11. The inlets which may be short pieces of pipe 30a and 30b of header 30 of suitable diameter are rotatably mounted in the clamp bearing 50 subtending from the slide element 42 of the horizontal-positioned adjusting means 28. Set screws 51 and threaded bolt 43 fix the inclination upon rotation of the header 30 to the desired inclination.

The position of the knives with respect to the pass line is controlled by means 28 through rotation of crank 52. As the handle 44 of crank 52 is rotated the slide element 42 travels horizontally toward or away from the pass line, thereby adjusting air knife assemblies 12. Horizontal movement is accomplished by rotation of the threaded section 52a of crank 52 within the threaded interior 42a of slide 42. Crank 52 is held in position by collars 52b on either side of slot 56 provided on U-shaped element 54. Slide members 53 guide and support the element 42 during travel thereof. The position of the knife with respect to the coating bath is adjusted by means 26 through rotation of crank 23 (see FIGURES l and 2). A simple 4 worm gear and rack combination, as shown, readily suffices for this purpose.

The positioning of the air knives used to accomplish control of the coating weight and distribution is critical to the successful performance of this function. A discussion of the factors involved and an explanation of the theories are presented in copending application Ser. No. 613,474, filed even date in the names of Darrell L. Hunter and James C. Siple, and such do not form a part of the present invention which is concerned with an apparatus useful to connect coating control. However, included as part of this apparatus as described above are means to adjust the vertical and horizontal positions of the air knives in the manner prescribed in the aforementioned copending application. In general, the knives should be positioned with their longitudinal axes horizontal and spaced from the pass line so as to provide a length of fluid travel to the strip of between about /2 and 1 /2 inches. The fluid nozzles may be inclined toward the substrate upwardly or downwardly at angles between about +20 above the horizontal to about 45 below a plane normal to the pass line. Angles of inclination of between +10 and 30 are most useful with angles of between 5 to l5 being preferred for galvanizing operations.

The knives advantageously are disposed between 8 and SO-inches above the coating bath surface. The lower positions within this range are desirable for operations at lower strip speeds. The critical positioning of the fluid streams referred to above and described in the aforementioned copending application requires that the jets from each knife overlap on the coated substrate at opposite sides thereof but do not impinge exactly opposite each other, i.e., the nozzles should not be positioned as mirror images of each other.

As has been shown, the curved knife in accordance with the invention includes orifice lips which curve horizontally or vertically away from the center of the knife at a predetermined radius and the length of the orifice defined by the lips lies along the arc of a circle of preselected radius, the chord of which is defined by a line adjoining the ends of the orifice. In a preferred embodiment wherein the orifice is curved as shown in FIGURES 4 through 7, the radius is selected so that in an orifice 48 to 54-inches long the ends of the orifice will rise 2 to 2 /2 inches above the center thereof. With such an arrangement, the radius of curvature of the orifice is about l3-feet although the actual radius of curvature of the top and bottom lips is slightly less and slightly greater respectively to achieve an orifice of uniform height throughout its length. It is noted from the cross sectional views of FIGURES S, 6 and 7 that the orifice lips 36 and 38 terminate in a common plane and that the convergent throat section 32 is formed to gradually and uniformly direct air entering through the rectangular opening in its flange 35 to the curved opening in its flange 41.

It is apparent from the configuration of the knives described herein that the orifice section prevents a curved profile, that is, fluid supplied to the header section issues forth at the orifice section at different distances from the pass line. This variation in distance achieved by the curvature in the air knife permits the formation of a" coating of convex cross section on the substrate.

As is evident from FIGURE 11 which depicts a knife having an orifice curvature as shown in FIGURE 4, the inclination of the increments of the fluid issuing from the orifice in this embodiment travel the same distance to the pass line. However, as the knife is rotated upwardly or downwardly from the completely horizontal position to define an angle A greater than 0, it is apparent:

(1) That the length of part of the increment of the fluid stream from the center of the knife increases at a greater rate than the length of travel of fluid jet increments from the ends of the knife which is also true for the embodiment shown in FIGURE 8;

(2) That the amount of the increase is a function of angle A; and

(3) That for a given angle A the length of travel of increments of fluid stream becomes progressively greater from edge to center. In effect, the stream from a curved lip knife in accordance with the invention, both embodiments of FIGURES 4 and 8, can be made to remove progressively more of the liquid coating toward the edges of the substrate by properly regulating the angle of the stream. Thus, the substrate can be provided with a coating which becomes progressively heavier toward the center, i.e., a coating of convex cross section.

Delivery of a uniform fluid stream from a long orifice slot requires maintenance of a uniform pressure across the entry end of the orifice which is achieved by providing a relatively high resistance of air flow to the orifice. The resistance to air flow is a direct function of the distance W, i.e., the length of the orifice section. Too high a resistance is wasteful of power. A length of orifice section or distance W of between 1 /2 to 2 /2 inches has proven satisfactory for a variety of applications.

I claim:

1. An apparatus for use in a system for controlling the Weight and distribution of a coating on a moving substrate which comprises means to move said substrate along a pass line, fluid nozzles adapted to be positioned on opposite sides of said pass line such that said substrate travels therebetween, means to support said nozzles, means to adjust the position of said nozzles with respect to said pass line, means to adjust the direction of said nozzles to the pass line; each of said nozzles comprising a header section, an orifice section and a convergent throat section therebetween, said header section adapted to receive fluid from fluid supply means, fluid diffusing means disposed between said header section and said orifice section; the terminal end of said orifice section being of such a shape and each of said nozzles being positioned with respect to said pass line such that fluid projected therefrom to the pass line traverses a greater distance to said pass line from the center of said orifice than from the edges thereof.

2. An apparatusaccording to claim 1 wherein said means to adjust the position of said nozzles with respect to the pass line includes means to vary the location of said nozzle in a plane normal to the pass line and means to vary the location of said nozzle in a plane parallel to the pass line.

3. An apparatus according to claim 1 wherein the terminal end of said orifice is of curvilinear configuration.

4. A fluid nozzle, adaptable for use in a system for controlling the weight and distribution of a coating applied to a moving substrate by the projection of streams of fluid against the coated substrate prior to solidification of said coating, which comprises a header section, an orifice section, and a convergent throat section therebetween, fluid diffusing means disposed between said header and orifice sections, said header section being adapted to receive fluid from a fluid supply means, said orifice section terminating in an orifice of curvilinear configuration which permits the nozzle to be positioned so that fluid projected therefrom against a substrate traverses a greater distance from the center of said orifice to said substrate than from the edges thereof.

5. A fluid nozzle in accordance with claim 4 wherein said orifice is curved in a plane normal to the centerline of a fluid stream adapted to be projected therefrom.

6. A fluid nozzle in accordance with claim 4 wherein said orifice is curved in plane of the same direction as the centerline of a fluid stream adapted to be projected therefrom.

References Cited UNITED STATES PATENTS 5/1915 Carter 118-63 X 8/1951 Miller 118-63 X 

